<div class="example row">
  <div class="col-md-3 panel">
    <div class="panel-heading">
      开始吧
    </div>
    <div class="panel-body" style="height: 100px">
      <button type="button" class="btn btn-danger btn-lg" data-toggle=
      "droppable" data-target=".droppable-target">拖动我</button>
    </div>
  </div>
  <div class="col-md-3 panel droppable-target">
    <div class="panel-heading">
      拖到这里
    </div>
    <div class="panel-body" style="height: 100px"></div>
  </div>
  <div class="col-md-3 panel droppable-target">
    <div class="panel-heading">
      拖到这里
    </div>
    <div class="panel-body" style="height: 100px"></div>
  </div>
  <div class="col-md-3 panel droppable-target">
    <div class="panel-heading">
      拖到这里
    </div>
    <div class="panel-body" style="height: 100px"></div>
  </div>
</div>
